Photoionization and photodissociation rates across a solar cycle

We calculate unattenuated photoionization and photodissociation rate coefficients using solar spectral irradiance (SSI) measurements from the TIMED/SEE mission (Woods et al., 2005) over a full solar cycle (2004–2014). These coefficients are parameterized by the solar activity index F₁₀.₇P, defined as F₁₀.₇P = ½(F₁₀.₇ + F₁₀.₇A), where F₁₀.₇ (10.7 cm solar radio flux) acts as a proxy for solar extreme ultraviolet (EUV) variability, and F₁₀.₇A represents its 81-day running average. Combining high-resolution (1 nm) SSI data with cross-sections from the PHIDRATES database (Huebner & Mukherjee 2015; http://phidrates.space.swri.edu/), we derive power-law relationships (j = A0[F₁₀.₇P]A1) for 48 reactions involving 12 species critical to Solar System atmospheres and cometary comae: H, H₂, OH, H₂O, O, O₂, C, CO, CO₂, N₂, N, and CH₄. 

Whereas prior estimates are limited to a subset of solar conditions (e.g., solar minimum or maximum), the photo rate coefficients derived here are broadly consistent with those results while extending them to span the full range of solar activity. This work establishes an efficient, observational approach to estimate photoionization rates requiring access to SSI or cross-section data. By utilizing F₁₀.₇P, a widely available and historical solar proxy, the power laws allow for easier modeling of atmospheric and cometary chemistry under diverse solar conditions. Applications include simulating tenuous exospheres (e.g., Mercury, Moon) and analyzing in situ data from outer planets and comets. The method is anchored in publicly accessible PHIDRATES cross-sections and TIMED/SEE SSI records (http://lasp.colorado.edu/home/see/data/daily-averages/level-3), ensuring utility for a variety of studies. Future work will address uncertainties for resolution-sensitive reactions and expand the species/reaction inventory, advancing our capacity to model solar-driven photochemistry throughout the Solar System.